Secretary of State Antony Blinken has reaffirmed the United States commitment to the support of Southeast Asian nations in a long-running dispute with China over sovereignty in the South China Sea.

The new top diplomat for the US made the commitment in a phone call with his Filipino counterpart, Teodoro Locsin, on Wednesday. Locsin had earlier lodged a formal diplomatic protest over Beijing’s decision to allow its coastguard, which is active in the disputed waters, to open fire on foreign vessels.

Blinken “pledged to stand with Southeast Asian claimants in the face of PRC pressure,” the State Department said in a statement following the call, referring to China by its formal name.

Blinken and Locsin agreed that the alliance between the US and the Philippines was “vital to a free and open Indo-Pacific region”.

Blinken “stressed the importance of the Mutual Defense Treaty for the security of both nations, and its clear application to armed attacks against the Philippine armed forces, public vessels, or aircraft in the Pacific, which includes the South China Sea”, the State Department added.

The US and the Philippines have been military allies for decades, but since Philippine President Rodrigo Duterte came to office in 2016 he has shifted the country closer to China despite Beijing’s more assertive claims in the South China Sea and construction of military bases and artificial islands in the disputed seas.

On Wednesday, Blinken sought to challenge that, saying the US rejects China’s maritime claims in the South China Sea “to the extent they exceed the maritime zones” permitted under international law.

Over the weekend, the US also sent a carrier group through the waterway to promote “freedom of the seas”. China is planning its own military drills this week.

‘Threat of war’

Last Friday, China passed legislation allowing its coastguard to use “all necessary means” to stop or prevent threats from foreign vessels, including demolishing structures built by other countries on reefs also claimed by China.

After initially dismissing the law as “none of our business”, Locsin changed his mind and lodged a diplomatic protest, calling the move a “threat of war”.

“While enacting law is a sovereign prerogative, this one – given the area involved or for that matter the open South China Sea – is a verbal threat of war to any country that defies the law; which, if unchallenged, is submission to it,” Locsin added.

The new Chinese law also permits coastguard to board and inspect foreign vessels in waters China considers its own, posing more problems given the scope of Beijing’s territorial claims in the South China Sea.

A Philippine fisherman watches a China Coast Guard vessel patrolling the disputed Scarborough Shoal in 2017 [File: Erik De Castro/Reuters]

China claims about 90 percent of the strategically important waterway, based on its controversial “nine-dash line”. An international tribunal in The Hague rejected the Chinese claim in 2016, following a case brought by the Philippines but China refuses to recognise the ruling.

China maintains a constant presence of coastguard ships hundreds of kilometres off its mainland, near disputed islands and often within the Exclusive Economic Zones (EEZ) of its neighbours. They have often been accused of acting aggressively, disrupting fishing boats and energy exploration.

Aside from the Philippines, Taiwan, Malaysia, Vietnam and Brunei also have claims on the sea.

Standing with friends, allies

Blinken’s call to Locsin is also seen as part of a renewed commitment to the region under the new administration of US President Joe Biden.

In multiple calls and statements, he and his top security officials have underscored support for Japan, South Korea and Taiwan, all traditional allies of the US.

On Wednesday, Biden told Japanese Prime Minister Yoshihide Suga that his administration is committed to defending Japan, including the Senkaku Islands, which are also claimed by China, which calls them the Diaoyu Islands.

State Department spokesman Ned Price has also warned China about menacing Taiwan after it repeatedly sent more than a dozen military fighters and bombers through the island’s air defence zone.

“We will stand with friends and allies to advance our shared prosperity security and values in the Indo-Pacific region – and that includes deepening our ties with Democratic Taiwan,” Price said in a statement.

“Our commitment to Taiwan is rock-solid.”